Submersible well pump assemblies (ESP) are commonly used to pump well fluid from oil wells. A typical ESP includes a pump and an electrical motor. The pump may be a centrifugal motor having a large number of stages, each stage comprising an impeller and a diffuser. Alternately, the pump may be another type, such as a progressing cavity pump. An ESP includes a pressure equalizer that couples to the motor to reduce a pressure difference between dielectric lubricant in the motor and the hydrostatic pressure of the well fluid. The ESP may include other components, such as a gas separator and additional motors and pumps in tandem. The various components are normally brought to a well site in separate modules, then secured together.
Generally the modules of the ESP are connected together by bolts that secure mating flanges. In some wells, a vertical section leads around a bend to an inclined or horizontal section. Inserting a lengthy ESP around the bend can cause stresses to the bolts.
It has been proposed instead of bolted flanges to employ threaded collars that are rotated to secure the various modules of the ESP. An example of a threaded collar arrangement is shown in U.S. Pat. No. 6,557,905. The threaded collar fits around a neck of an adapter of one of the modules and engages threads on the adapter of the other module. One concern is the possibility of vibration during operation causing the collar to loosen or back off from the threaded engagement.